The historic civil nuclear deal between the US and India is running into serious difficulties over New Delhi’s insistence that the Bush administration rewrite elements of the law enacted by Congress last year.

US state department officials say that India’s negotiating stance risks unravelling the deal, which gives India unprecedented access to nuclear fuel without requiring it to sign up to the nuclear non-proliferation treaty. The agreement was enacted by large margins on Capitol Hill in December and marked a dramatic breakthrough following a history of often tense relations between the two countries.